This invention relates to the assembly and packaging of microelectronic devices, including particularly the contact bonding and assembly of integrated semiconductor circuits.
Various methods have been proposed for providing electrical connections between the ohmic-contact areas of an integrated microcircuit and the external package leads. The most common method in current use involves the thermocompression bonding of extremely fine wires to the points to be interconnected. In accordance with this technique, a 14-lead device, for example, requires 28 separate bonding steps, each requiring a careful positioning of the partially assembled device in the bonding apparatus.
The industry has recognized for some time that it would be desirable to eliminate the time and expense of wire bonding. Considerable attention has been devoted to the expedient of simply extending the internal portions of the package leads and tapering the lead ends to provide bonding tips which are small enough for attachment directly to the bonding pads of the semiconductor structure. This approach has not been successful, primarily due to the fundamental difference in structural specifications required for external package leads as compared with the specifications required of internal leads bonded to the ohmic-contact areas of a microcircuit.
For example, the use of external package leads made of 10-mil Kovar has become a standard practice for many devices. Efforts to bond 10-mil Kovar leads directly to the electrodes of an integrated circuit have proved disappointing. High-speed techniques for gold or aluminum wire bonding, such as thermocompression bonding and vibratory pressure welding, do not readily produce a reliable bond when applied to leads as thick as 10-mils, or when applied to metal leads which are less ductile than gold, aluminum, or copper, for example. Even when acceptable bonds are initially formed using a 10-mil lead frame, the leads are very susceptible to inadvertent detachment from the die as a result of subsequent stresses introduced by normal handling and incidental flexing of the assembly.
It has also been proposed to replace bonding wires with individual rigid metal clips for interconnecting the bonding pads of the semiconductor circuit with the external leads. This approach may be advantageous for some applications, but it has not been found to reduce the cost of assembly substantially.
It is also known to deposit a metallic pattern of interconnecting leads on a ceramic base or other support, such that an integrated circuit die having built-up electrodes may be inverted and contact bonded face down to the metallic pattern. This approach is objectionable because of high costs, and because the bonding sites are hidden from visual inspection. Therefore, any defective bonds will escape detection until the device can be electrically tested.